WASHINGTON (Reuters) - Lawmakers announced a renewed push on Wednesday to make sweeping changes in the way the military handles complaints of sexual assault, but they face a tough fight attracting the 60 votes they will likely need to get their plan through the Senate.

Democratic Senator Kirsten Gillibrand of New York said 46 senators - 38 Democrats and eight Republicans - support her proposal to remove the power to decide whether to try sexual assault cases from the military chain of command and put it in the hands of an independent military prosecutor.

"It is a pro-military piece of reform that will enhance our military readiness," Gillibrand said at a news conference with seven other senators, a victim of assault, advocates and retired military officers to announce the effort to pass the proposal this month.

But she has only about two weeks to win over at least 14 more senators before the measure comes before the full Senate as an amendment to the 2014 National Defense Authorization Act.

Gillibrand's plan faces stiff opposition from the Pentagon, where top military leaders argue that prosecutions must remain with military commanders to maintain good order and discipline.

"The Department believes we need more commander involvement, not less, in order to solve this problem," said Army Lieutenant Catherine Wilkinson, a Pentagon spokeswoman. "Commanders lead by example, they set and enforce standards and help to increase victim confidence in the sexual assault prevention and response program."

Gillibrand's plan also faces resistance in the Senate, where lawmakers including fellow Democrats like Carl Levin, the powerful chairman of the Senate Armed Services Committee, say the military's rules for handling sexual assault complaints should be reformed, but would not go as far as Gillibrand.

Levin supports a plan, which is expected to pass as part of the defense authorization bill, to let military commanders continue deciding whether to bring sexual assault cases to trial, but add levels of review by more senior leaders.

An annual Pentagon study released in May estimated that incidents of unwanted sexual contact, from groping to rape, jumped by 37 percent in 2012 to 26,000 cases from 19,000 the previous year.

The Pentagon has also been struggling to deal with a spate of high-profile cases of sexual assault, including some involving personnel charged with combating the crime.

CONTROVERSY ENSNARES NOMINEE

President Barack Obama's nominee to be undersecretary of the Navy, Jo Ann Rooney, has also been caught up in the controversy. Gillibrand and another Democrat who supports her legislation, Senator Barbara Boxer of California, are blocking her nomination in anger about Rooney's comments on her reservations about the plan.

"I believe the impact would be decisions based on evidence rather than the interest in preserving good order and discipline," Rooney said at her confirmation hearing last month.

Gillibrand said the U.S. legal system is based on evidence. "Why isn't that good enough for our service members?" she asked at the news conference, explaining her hold on the nomination.

The Senate is expected to begin debate on the defense bill during the week of November 18. Several lawmakers said they expect opponents of Gillibrand's plan would use Senate procedural rules to ensure that it needs 60 votes - not a majority of 51 - for passage.

Gillibrand and other supporters of her proposal argued that it will enhance military readiness and strengthen morale.

"When young adults commit to serving their country and defending our freedoms, they deserve to know their own rights will be protected, including access to justice," Republican Senator Chuck Grassley of Iowa told the news conference.

Ariana Klay, a former Marine officer and Iraq War veteran who was a victim of sexual assault, told how military rules forced her to seek justice from a commander who said she deserved bad treatment because she wore makeup and running shorts.

She said she faced a year of retaliation after filing a complaint before her accused rapists' trial, in which she endured 15 hours of degrading questioning on the witness stand.

"Even an alleged terrorist on U.S. soil has more protection than a U.S. Marine," said her husband, Ben Klay, also a former Marine, his voice breaking as he discussed his wife's case.

(Additional reporting by David Alexander; Editing by Mohammad Zargham)